In the manufacture of steel in the open hearth furnace, the usual method of operation is to charge the furnace vessel with limestone and/or lime, steel scrap and pig iron, in that sequence. In the electric furnace, no pig iron is used. The metallic charge is then melted by heat of an extraneous fuel, such as oil or gas in the case of the open hearth furnace, or electricity in the case of the electric furnace.
Lime is an important and major constituent of steel-making slags and there has always been a tendency to charge more than enough lime into the furnace vessel in order to insure that enough lime is present. However, the lime has a high melting point (2570 degrees Centigrade) and the furnace vessel and its lining containing the melt will not stand up at the temperature needed to fluidize lime.
Accordingly, in the present steel-making process, other minerals, such as alumina, fluorspar, iron oxide and the like, have been added to lower the melting point of the lime. As the lime becomes fluid, it rises through the molten metal in the furnace vessel and goes to the top thereof, and in the process combines with undesirable elements in the pig iron or scrap, to form the slag.
Because of the fact that lime is so difficult to fluidize, and because an excess is frequently used in the furnace charge, there is a tendency for lime build-up on the furnace bottom which, in time produces undesirable results, as will be pointed out hereinafter. I have discovered that lime build-up may be overcome by periodically adding to the vessel, after the melt has been tapped, one or a combination of the minerals used to reduce the melting point of the lime in the steel-making process so as to fluidize the lime and permit it to be removed through the furnace tap hole .